Anti-reflection coatings are used extensively in optical and/or opto-electrical applications where it is often necessary to decrease reflections at an interface, such as at the boundary between air and glass.
The simplest anti-reflection coating consists of a single layer of a transparent dielectric material deposited directly on a substrate. The layer of transparent dielectric material is selected such that it has an index of refraction, n, that is less than the index of refraction of the substrate, and such that it has a thickness, d, that allows the optical thickness (n times d) to be about one quarter of the central wavelength of the spectral region for which the reflectance is to be reduced.
More complex anti-reflection coatings are made by depositing two or more layers of transparent dielectric materials on a substrate. For example, according to one type of anti-reflection coating, a first layer having an index of refraction higher than that of the substrate is deposited on the substrate such that its optical thickness is about one quarter of the central wavelength, while a second layer having an index of refraction lower than that of the substrate is deposited on the first layer such that its optical thickness is also about a quarter of the central wavelength. This type of anti-reflection coating is often referred to as a V-coat design because it generally achieves a zero reflectance at the central wavelength, with sharply increasing reflectance at either side of the central wavelength.
Multi-layer anti-reflection coatings that are more suitable for broadband applications generally have at least three dielectric layers of alternating high and low refractive index materials stacked together. For example, one particularly well-known broadband anti-reflection coating includes a first layer formed from a material having a high index of refraction and having an optical thickness of about one-eighth of the central wavelength deposited on the substrate, a second layer formed from a material having a low index of refraction and having an optical thickness of about one-eighth the central wavelength deposited on the first layer, a third layer formed from a material having a high index of refraction and having an optical thickness of one half the central wavelength deposited on the second layer, and a fourth layer formed from a material having a low index of refraction and having an optical thickness of one quarter of the central wavelength deposited on the third layer. The optical thicknesses of the first and second layers are selected to provide a combined optical thickness that is about one quarter of the central wavelength of the spectral region for which the reflectance is to be reduced such that first and second layers form what is commonly referred to as a simulated quarterwave layer.
As illustrated above, the surface layer of most anti-reflection coatings is selected to have a low index of refraction, which is preferably lower than that of the substrate, and a quarterwave optical thickness so as to minimize reflectance from the surface of the coating. While this is feasible in most instances, design challenges often arise when the anti-reflective coating is designed to be electrically conductive.
Conductive anti-reflection coatings have found widespread use in numerous applications where electrical conductivity and high optical transparency are needed. For example, conductive anti-reflection coatings are commonly used for electro-magnetic interference (EMI) shielding in cathode ray tubes (CRTs), as electrodes in liquid crystal displays (LCDs), and in thin film resistive heaters. In these applications, the electrical conductivity is typically provided by a layer of transparent electrically conductive material, such as indium tin oxide (ITO).
The index of refraction of most transparent electrically conductive materials, however, is relatively large. It is this high index of refraction that complicates the design of conductive anti-reflection layers. For example, as discussed above, it is generally preferred that the surface layer of an anti-reflection coating be a quarterwave dielectric layer having a low index of refraction so as to minimize reflectance from the surface of the coating. In contrast, it is also preferred that the conductive layer, which generally has a high index of refraction, be fabricated as the surface layer to reduce the high contact resistance associated with a surface quarterwave dielectric layer.
In practice, most conductive anti-reflection coatings are arranged such that the conductive layer is buried below a quarterwave dielectric layer. For example, in the above-mentioned four layer design for broadband applications, the conductive layer typically corresponds to the halfwave layer. This arrangement improves the anti-reflection properties of the coating, protects the conductive layer, and provides more flexibility in varying the thickness of the conductive layer. To reduce the high contact resistance associated with the surface quarterwave dielectric layer, the electrical connections are typically made either by penetrating the surface dielectric layer (e.g., using an ultra-sonic soldering or welding procedure) or with direct contact to the conductive layer in regions where the dielectric layer is absent (e.g., using a mask during production to keep the conductive surfaces free of dielectric material).
Unfortunately, these methods of reducing surface contact resistance significantly increase costs and lengthen production times. In particular, ultra-sonic soldering or welding procedures are known to cause a bottleneck in the production of conductive anti-reflection coatings. Similarly, using a mask to create dielectric-free regions on the conductive layer is associated with expensive precision tooling and additional time-consuming steps. For example, when the conductive and dielectric layers are deposited by vacuum evaporation, the latter process includes the additional time-consuming steps of venting the chamber after the conductive layer has been deposited, precisely applying the mask, and re-evacuating the chamber prior to depositing the surface dielectric layer.
It is an object of the instant invention to provide a conductive anti-reflection coating with low surface contact resistance.
It is another object of the instant invention to provide a conductive anti-reflection coating with low surface contact resistance that is fabricated without significantly increased costs and lengthened production times.